The common Nettle (_Urtica dioica_) is the type of this division; and we are so accustomed to consider it a noxious weed, that few persons are aware of the elegance of its flowers, which are disposed in drooping panicles. The male flowers have their calyx divided into four sepals; and they have four stamens, the anthers of which open with elasticity, and when they spring back, the pollen, which is very abundant, is discharged with such force that it may be seen on a fine day in summer rising like a mist or light cloud over the plants. The stamens, after they have discharged their pollen, lie extended and curved back over the segments of the calyx, as shown at _a_ in _fig._ 72. The female flowers have only two segments to the calyx. They have no style, and the stigma, when highly magnified, will be found divided into numerous segments, as shown at _b_; the seed-vessel is a nut, which has a shell and kernel, the latter being the seed. The leaves are simple, cordate, opposite to each other, and furnished with stipules. They are rough on the surface, and covered with glandular hairs or stings. These hairs are hollow, with a cell at the base filled with a peculiarly acrid liquid, and tapering upwards so as to form a narrow tube, ending in a sharp point. When the point of the sting enters the skin, the pressure compresses the cell at its base, and the liquid it contains is forced up the tube and injected into the wound. The stem is quadrangular, and its fibres are so tough, that when separated from the pulp by maceration, they may be spun into yarn. The young shoots when boiled are very good to eat. The Roman Nettle (_U. pilulifera_) differs from the common kind in having the male flowers in loose panicles, and the female ones in compact pill-like heads, whence the specific name. The sting of this nettle is worse than that of the common kind. [Illustration: FIG. 73.—HOP (_Humulus Lupulus_).]